Wing Resonances in a New Dead-Leaf-Mimic Katydid 1 2 3 2 (Tettigoniidae: Pterochrozinae) from the Andean Cloud Forests 4 5 6 3 7 8 1 1 2 1 9 4 ANDREW BAKER , FABIO A
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*Manuscript Click here to view linked References Wing resonances in a dead-leaf-mimic katydid 1 1 Wing resonances in a new dead-leaf-mimic katydid 1 2 3 2 (Tettigoniidae: Pterochrozinae) from the Andean cloud forests 4 5 6 3 7 8 1 1 2 1 9 4 ANDREW BAKER , FABIO A. SARRIA-S , GLENN K. MORRIS , THORIN JONSSON 10 11 5 AND FERNANDO MONTEALEGRE-Z1* 12 13 14 6 15 16 7 1School of Life Sciences, Joseph Banks Laboratories, Green Lane, Lincoln, LN6 7DL, UK 17 18 8 2Department of Biological Sciences, University of Toronto Mississauga, 3359 Mississauga 19 20 21 9 Road North, Mississauga, Ontario, Canada, L5L 1C6 22 23 10 24 25 26 11 Running title: Wing resonances in a leaf-mimic katydid 27 28 12 29 30 31 13 * Corresponding author. E-mail: [email protected] 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Wing resonances in a dead-leaf-mimic katydid 2 14 Abstract 1 2 3 15 Day-camouflaged leaf-mimic katydids Typophyllum spp. have a remarkable way of evading 4 5 16 predators as male and female forewings appear as bite-damaged leaves complete with necrotic 6 7 17 spots. As in all other katydids, males produce sound signals to attract females by rubbing their 8 9 10 18 forewings together. The biophysical properties of these special leaf-like forewings remain 11 12 19 obscure. Here we study the wing mechanics and resonances of Typophyllum spurioculis, a 13 14 15 20 new species of leaf-mimic katydid with a broad distribution in the Andes from Western 16 17 21 Ecuador to the middle Central Cordillera in Colombia. This species performs an unusual 18 19 20 22 laterally directed aposematic display, showing orange spots that simulate eyes at the leg base. 21 22 23 At night, males are conspicuous by their loud, audible calling songs, which exhibit two 23 24 25 24 spectral peaks at ca. 7 and 12 kHz. Using micro-scanning laser Doppler vibrometry we find 26 27 25 the effective sound radiators of the wings (speculae) vibrate with three modes of vibration, 28 29 26 two of which include the frequencies observed in the calling song. Remarkably, this 30 31 32 27 resonance is preserved in the parts of the wings mimicking necrotic leaves, which are in 33 34 28 theory not specialised for sound production. The eyespot function is discussed. 35 36 37 29 38 39 30 Keywords: Katydid, Bush-cricket, Mimetism, Stridulation, Resonance 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Wing resonances in a dead-leaf-mimic katydid 3 31 Introduction 1 2 3 32 Katydid species of the genus Typophyllum are masters of camouflage and mimicry. Both 4 5 33 sexes exhibit remarkable forewing adaptations, resembling chewed leaves living and dead, 6 7 34 with necrotic spots, marginal feeding ‘bites’ or even skeletonized ocellata (Braun, 2015b; 8 9 10 35 Nickle and Castner, 1995). During the daytime, males and females rest on trees and bushes 11 12 36 overlooked by diurnal predators. Individuals adopt postures making them almost 13 14 15 37 indistinguishable from the surrounding foliage: the head is applied to a plant branch and the 16 17 38 insect’s long antennae projected forward and down along the axis of the same branch (Fig. 1). 18 19 20 39 All known species are nocturnally active, when the males produce loud calling songs to 21 22 40 attract females. (Braun, 2015a; Braun, 2015b; Montealegre-Z and Morris, 1999). Females are 23 24 25 41 larger than males and their tegmina are not only comparatively larger, they also differ in 26 27 42 venation pattern and shape from the male wings (Braun 2002; Nickle, 1992). 28 29 43 During a collecting expedition in 1996 in Ucumarí (Colombian Andean cloud forests, see 30 31 32 44 methods), GKM and FM-Z discovered a male of a curious Typophyllum species, which they 33 34 45 later briefly mentioned (without description) (Morris and Montealegre-Z, 2001). In the same 35 36 37 46 article they also figure an analysis of its calling song: a spectrum with two maximum 38 39 47 harmonically-related peaks at around 7 and 13 kHz. More than 20 years later, FS, AB, and 40 41 42 48 FMZ discovered the same species in the montane forest of Santa Lucía, a cloud forest reserve 43 44 49 located in the Pichincha province of Ecuador. The species is very abundant in this locality, 45 46 50 with males dominating the audible acoustic space in the nights. 47 48 49 51 Male katydids produce calling songs to attract potential mates by rubbing their forewings, a 50 51 52 mechanism known as stridulation (Gwynne, 2001; Robinson and Hall, 2002). A sharp lobe on 52 53 54 53 the right wing (the scraper) is swept upon a vein lined with precisely arranged cuticular teeth 55 56 54 on the left wing. A large specular wing cell (adjacent to the scraper) on the right wing, the so- 57 58 59 55 called mirror, subsequently couples the low amplitude vibrations, produced by this initial file- 60 61 62 63 64 65 Wing resonances in a dead-leaf-mimic katydid 4 56 scraper interaction (Bailey and Broughton, 1970; Broughton, 1964; Morris, 1999; Morris and 1 2 57 Pipher, 1967; Robinson, 1990) to the air. 3 4 5 58 A particular characteristic of the acoustic signals in the genus Typophyllum is that the calls of 6 7 59 the males exhibit a very sharp spectrum with high Q and that most species known so far 8 9 10 60 communicate at about 20 kHz (Braun 2002; Braun, 2015a; Braun, 2015b; Montealegre-Z and 11 12 61 Morris, 1999; Morris et al., 1989) (Q=quality factor, which describes the properties of a 13 14 62 damped resonator or oscillatory system (Bennet-Clark, 1999b). Would the presence of two 15 16 17 63 harmonically related peaks in the call of our new species Typophyllum spurioculis (here 18 19 64 described) involve two independent resonators in the wings, or could not the same speculum 20 21 22 65 or another wing area vibrate to produce harmonics? 23 24 66 In katydids, the stridulatory fields of left and right tegmina are asymmetric. The stridulatory 25 26 27 67 area of the left forewing where the file resides, is usually damped to sound vibration, while 28 29 68 that of the right wing is heavily involved in sound radiation (Chivers et al., 2017; 30 31 69 Montealegre-Z and Postles, 2010; Sarria-S et al., 2016; Sarria-S et al., 2014). In contrast to 32 33 34 70 katydids, in crickets and grigs (with morphologically symmetric or nearly symmetric wings) 35 36 71 both left and right wings contribute more or less equally to sound radiation (Chivers et al., 37 38 39 72 2016). In theory, the observed wing asymmetry in katydids will affect the radiating sound 40 41 73 field, having a negative effect on the output power. In other words, a sound radiator involving 42 43 44 74 symmetric wings is analogous to having two synchronised speakers. Therefore, katydids, 45 46 75 having only one functional sound radiator, sacrifice one speaker for sound radiation. Katydids 47 48 have resolved this problem by developing additional morphological adaptations to amplify 49 76 50 51 77 their signal (Bennet-Clark, 1998). For example, special wing positions that favour signal 52 53 78 broadcasting (Stumpner et al., 2013), wing inflations (Hemp et al., 2013), and exaggerated 54 55 56 79 pronotal extensions that work as resonators (Jonsson et al., 2017; Morris and Mason, 1995). 57 58 80 However the wings of male Typophyllum, seem not to have special adaptations for resonance 59 60 61 62 63 64 65 Wing resonances in a dead-leaf-mimic katydid 5 81 (Braun, 2015a; Montealegre-Z and Mason, 2005; Morris et al., 1989). While the effective 1 2 82 stridulatory area in the right wing, the mirror, is only a small oval portion of the entire large 3 4 5 83 leaf-like wing (Braun 2002; Braun, 2015a; Montealegre-Z and Morris, 1999), the calls of the 6 7 84 male are very loud, suggesting that these insects might use a combination of resonance and 8 9 10 85 subalar space to enhance the call output. The biophysical properties of the wing structure used 11 12 86 for camouflage and not involved in sound radiation remains elusive. 13 14 87 In this paper we use micro-scanning laser Doppler vibrometry to investigate the wing 15 16 17 88 resonances associated with the production of the species’ acoustic signals, and the resonances 18 19 89 associated with the wing regions involved in camouflage. We also describe a new species of 20 21 22 90 Typophyllum, and document the variation of wing morphology and venation across 23 24 91 individuals. 25 26 27 92 28 29 93 Materials and methods 30 31 32 94 Field sampling 33 34 95 Fieldwork was conducted in Colombia, at Parque Regional Natural Ucumarí (5º N, 76º W). 35 36 37 96 This is located in the eastern slope of the Central Cordillera, above the tiny hamlet El Cedral, 38 39 97 22 km east from Pereira in the Department of Risaralda. A narrow glaciated valley rises into 40 41 42 98 the montane rainforest over elevations of 1850-2600 m, and high waterfalls enter along the 43 44 99 sides.